The present invention relates to the production of highly pure steam. It finds particular application in conjunction with pure steam generators using falling-film evaporator and rising channel technology for separating water droplets and impurities from the steam and will be described with particular reference thereto.
Most water-borne contaminants do not vaporize as water is boiled and therefore do not pass to the condensate or distillate of a still. The removal of contaminants involves a phase change of the water to water vapor, which typically leaves the impurities behind. However, in the boiling process, small droplets of water still carrying impurities can be carried with the steam. Mineral contaminants and biological contaminants, such as pyrogens, can be suspended in the water droplets carried with the steam. When the water droplets are removed from the steam, the resultant pure steam can have all impurities removed down to a range of 10 parts per trillion.
Highly purified steam is used for various medical purposes, such as the production of pharmaceuticals, demanding sterilization applications, and the production of water for injection, i.e., sterile water with no added substances that is sufficiently pure and free from contamination as to be used in the preparation of parenteral solutions and pharmaceuticals.
One technique for the production of pure steam utilizes a falling-film evaporator. A vertical bundle of evaporation tubes encloses a heating jacket which, in turn, is enclosed by an outer shell defining an annular space between the heating jacket and the outer shell. Water is fed into the evaporator tubes at their upper ends and flows down the inner surface of the tubes evaporating and forming steam. The steam emerges at the lower end of the tube bundle. The flow of steam makes a 180xc2x0 turn and flows upward in the space between the heating jacket and the annular shell. Fins attached to the outer surface of the heating jacket define a spiral path which leaves a narrow gap between their edges and the inner surface of the shell. As the steam flows upward through the spiraling path, water droplets in the evaporation product are driven to the outer shell by centrifugal force. The droplets adhere to the outer shell and form a film of water flowing downward and forming a pool of liquid at the bottom of the device. From there, a stream of water proportional to the amount of pure steam produced is withdrawn as a reject stream. The water phase in the evaporation product tends to be rich in impurities which are enriched in the reject stream. Pure steam emerging from the top of the spiral path is fed to steam consumption points or to a condenser for producing highly pure water. See, U.S. Pat. No. 3,875,017 issued Apr. 1, 1975 to Saari and Huhta-Koivisto.
In another pure steam generator, the evaporation product emerges from the lower end of the tube bundle and is brought into a circular motion by baffle fins at the bottom of the device. The evaporation product rises in a surrounding annular space which narrows toward the top. A spiral path is arranged at the top of the rising space by fins that reach the inner surface of the outer shell. The droplets accelerated by centrifugal force are collected in an annular channel above the spiral path. A separate tube returns the resulting water phase to the bottom of the device rather than relying on downward flowing reject phase along the inner or outer shell.
In the prior art devices, the steam paths are not easily accessible. The structure of the separator units is closed. Access for corrosion inspection and maintenance is difficult. The inner structure is integral with the pressure vessel shell. The structures cannot be altered without going through the tedious procedure involved with pressure vessel construction.
The present application presents a new and improved pure steam generation technique which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a falling-film evaporator includes a vertical tube bundle and a heating jacket. The evaporation product emerges from a lower end of the tube bundle.
A device for separating water droplets and impurities from the evaporation product includes a length of downpipe through which the evaporation product from the vertical tube bundle initially flows. The evaporation product makes a 180xc2x0 turn and enters a rising channel between an outer surface of the down pipe and an inner surface of an intermediate shell. The down pipe is preferably tapered forming a funnel. Spiral fins disposed in an upper part of the rising channel set the evaporation product in an upward spiraling circular motion.
The intermediate shell includes at least one opening or outlet slit at the periphery of the spiral path through which droplets are propelled by centrifugal force. A cooled surface of an outer shell is disposed outside the openings to insure that droplets and steam passing through the openings is condensed. Steam condensing on the inner surface of the outer shell creates a radial outward stream that carries steam, water droplets, and impurities on such inner surface. A water film is formed which flows down the surface in the space between the outer and intermediate shells. A pool of water is formed at the bottom of the unit submerging the lower edge of the intermediate shell. A controlled stream of reject water is withdrawn from the pool. The pure, dry steam leaves the upper end of the spiral path and exits the device.
In accordance with another aspect of the present invention, the inner parts are detachable for easy maintenance and cleaning.
One advantage of the present invention is that it enhances the separation of water droplets and impurities in the rising channel of a falling-film evaporator.
Another advantage of the present invention is that the entire apparatus need not be pressure vessel certified.
Another advantage of the present invention is that it facilitates maintenance and cleaning.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.